The present invention relates to an improvement in an ink jet printing apparatus in which at least one nozzle ejects a jet of ink subjected to supersonic vibration and a charging electrode selectively charges the ink at a position where the jet separates into droplets whereupon deflecting electrodes deflect the charged droplets of ink causing them to impinge on a sheet of paper. More particularly, the present invention relates to a new ink jet printing apparatus of the type described which enhances the quality of reproduction by minimizing the fluctuation of deflection or distortion which may result from a change of ambient conditions or that of operating conditions.
Generally, an ink jet printer of this type is so constructed as to charge ink droplets in accordance with print data and deflect them so that the ink droplets impinge on a sheet of paper to print out desired data thereon. Each of the flying ink droplets generates a stream of air behind it. When an ink droplet enters a stream of air generated by the immediately preceding ink droplet, the aerodynamic resistance acting on the following ink droplet is reduced to such a degree that the distance between the adjacent ink droplets may become smaller or even zero. The result is the distortion to an image or character printed out on the sheet. The distortion also results from the Coulomb's force which would act between the adjacent charged ink droplets to affect their distance. Additionally, each charged ink droplet ahead of one which is about to be charged might reduce the expected amount of charge on the latter thereby further promoting the distortion.
One of solutions to such a problem is disclosed in U.S. Pat. No. 3,946,399. The technique taught by this U.S. Patent is to make up for the mutual influence of adjacent charged ink droplets due to the Coulomb's force and the deflection due to the aerodynamic resistance by detecting a print pattern in advance and compensating a charging amount in accordance with the detected pattern.
The technique mentioned above, however, will become ineffective when the number of deflection steps is substantial such as 32 steps, for example. With the increase in the number of deflection steps, the distance between the adjacent ink droplets is made shorter to promote the distortion. Additionally, the flight time of an ink droplet and, therefore, the amount of distortion are dependent on the amount of deflection. It follows that the distortion cannot be adequately compensated for unless compensated in conformity to a specific number of deflection steps.
Meanwhile, it has been customary to supply an ink ejection head with an ink under predetermined pressure by means of a constant pressure pump or the like. However, difficulty has been experienced in so supplying the ink due to the scattering in nozzle diameters of ink jet heads and because the filter tends to be stopped up to invite a pressure loss. Even if a supply of ink under constant pressure could be realized, any change of temperature adjacent the nozzle would change the viscosity of ink around the nozzle. This would vary the velocity or kinetic energy of a flying ink droplet and, thereby, the deflection and distortion.
Thus, a temperature control has heretofore been carried out to control the ink temperature around the nozzle to a predetermined level. Such a control has still involved a problem concerning the buildup time of the printer, because the printer usually requires about one minute to have the temperature around the nozzle elevated, for example, from 5.degree. C. up to a desired level (25.degree. C).